MN cars should generally get tubular diaphragms while Amfleets have a different style with a metal outer band, both of which are available as non-operating styrene parts from Train Station Products.
You can make semi-operating tubular diaphragms using black rubber heat-shrink tubing - a 1/8" diameter should be the right size. Be careful when cementing these tubes to the model, though, as superglue can cause the rubber to become rigid in places.
It's much trickier to make operating Amfleet diaphragms yourself, unless you want to modify the ends of your cars the way Walthers does. This way, the solid plastic diaphragms can move in a slot that is molded into the car end, and they are sprung from behind. I started experimenting with using the old folded-paper diaphragms that Walthers sells, making new styrene striker plates and attaching them to the paper bellows for a somewhat flexible diaphragm. Like I said, though, it's tricky and I haven't felt like getting it down to a science so I can crank out several more pairs.
Detail Associates makes almost any type of coupler lift bar that you would want, and they all come as formed-wire parts with separate brackets so they are fully moveable. Don't forget to search the Walthers catalog or website, since these parts are very easy to find.
You could add rotating bearing caps on locomotives by drilling through the sideframes and extending the axles using styrene or metal rod. Details West and Train Station Products make bearing caps that could be modified to fit. However, you're really taking a chance here because you could seriously impair the engines' operating performance if you're not careful. I recommend against trying it unless you're confident you can pull it off, and if that's the case I recommend you buy a pair of the Kato or Athearn Genesis freight car trucks that include rotating bearing caps, so that you can study the way they got it to work.
